This is the ninth of a series of reviews, looking back at a year of science according to topic and theme. This one celebrates science as enlightenment. Some of these discoveries make us consider the world in a new light, some act as timely reminders of things we already know, and others complicate a previously simple picture. They tell us that no matter how much we think we know, it’s all a little more complicated than that.

The OXTR gene acts as a dock for oxytocin, a hormone frequently billed as a “love hormone”. Among Americans, a specific “G” version of OXTR makes carriers more likely to seek emotional support from their friends. But for Koreans, whose culture frowns on seeking support from one’s social circle, the opposite happens. People with the G version are less likely to turn to their peers in times of need. To top it all off, both of these effects only showed up when people experienced a lot of stress. The study reminds us that the same stretch of DNA can lead to very different deeds, depending on individual circumstances. Environments and cultures set the stage on which genes expresse itself.

Seahorses and pipefish took a big leap from models of fatherhood to vampiric cannibal abortionists. The females lay their eggs into a pouch in the male’s belly and he carries the developing babies to term. They may seem like a shoe-in for a Dad-of-the-year award but two studies this year showed that males can turn into cannibals. They can absorb the young in their pouch for their own nutrition. What’s more, they’re more likely to do this if they’ve mated with an unattractive female.

The oceans are in trouble. Every year, a mountain of research spells trouble for watery wildlife, as climate change heats the oceans and makes their waters more acidic. This year, one of the most alarming studies showed that tiny green creatures called phytoplankton are disappearing. This couldn’t be more important. Phytoplankton are the bottom of the ocean’s food web, and they produce much of the planet’s oxygen. Their numbers have fallen by around 1% per year over the last century as the oceans have become warmer, and if anything, their decline is getting faster. Our blue planet is becoming less green with every year.

Newspapers are full of examples of powerful people behaving badly, from the diplomats in the Wikileaks cables to the peers in the UK’s expenses scandal. Power, it is said, corrupts, and Joris Lammers has solid evidence for this. He showed that powerful people are more likely to behave immorally than those with less power, but paradoxically less likely to tolerate immorality in other people. They frowned more strongly upon speeding, tax-dodging or keeping stolen goods, but were more lenient about doing it themselves. Even thinking about the feeling of power can trigger these double standards.

Tamiflu is our main line of defence against the H1N1 flu virus but seasonal strains of H1N1 have been shrugging it off since 2007. Now we know why. H1N1 has developed a mutation called H274Y that prevents Tamiflu from sticking to its surface. On its own, the mutation also weakens the virus, and scientists wrongly believed that it wouldn’t cause a problem. Instead, it spread like wildfire because of H1N1 strains had already picked up other changes that compensated for H274Y. These innocuous “permissive mutations” sowed the seeds of resistance, eventually allowing the virus to pick up the H2747 mutation at no cost to itself. It reminds us that evolution works in unpredictable ways, and without any greater plan in mind.

Do national parks and reserves do more harm than good? Some argue that these protected areas save wildlife at the expense of poor people, who can’t use the land for farming or resources. But such objections are unfounded. By actually comparing similar communities on a small scale, Kwaw Andam found that protected areas in Costa Rica and Thailand have actually helped to alleviate poverty. The areas bring opportunities from business and investments, promote tourism, and improve local infrastructure.

In 1980, smallpox was declared extinct. Victory in hand, countries around the world discontinued their vaccination programmes. That was a mistake. The smallpox vaccine also reduced the risk of contracting a related illness called monkeypox and without the vaccine, monkeypox is on the rise in Africa. In the Democratic Republic of Congo, the virus has become 20 times more common in the last 30 years. Resuming vaccination isn’t the answer for a politically unstable area; active surveillance is a better bet. Ironically, such a programme was set up in the 80s, and abandoned because the disease was deemed to be no threat to public health.

There are countless examples of people holding on to their beliefs in the face of overwhelming evidence to the contrary. That’s not unexpected. This year, three experiments showed that when people’s confidence in their beliefs is shaken, they become stronger advocates for those beliefs. Contrary evidence creates a sense of intense discomfort as people try to cope with conflicting ideas. People go to great lengths to reduce this “cognitive dissonance”; some try to do it by mustering support for their ideas.

This year, Archbishop Desmond Tutu and !Gubi, a tribal Bushman elder, became the second and third African people to have their entire genomes sequenced. !Gubi’s genes are vastly different to that of three other Bushmen from neighbouring areas. Pick any two and peer into their genomes and you’d see more variety than you would between a European and an Asian. This diversity reveals just how important it is to include African people in genome sequencing projects. The continent is the birthplace of humanity and its people are the most genetically diverse on the planet. To understand human genetics without understanding Africa is like trying to learn a language by only looking at words starting with z.

The ability to reset adult cells into a stem-like state was one of the biggest advances of the last decade. The reprogrammed cells have the potential to produce any of the various cells in the human body, with obvious implications for medicine. But there’s a catch. This year, Kitai Kim found that the cells retain a memory of their past specialities. A blood cell, for example, can be reverted back into a stem cell, but it carries a record of its history. It keeps molecular Post-it notes that annotate their DNA and affect how it’s used. This constrains its future, making it easier to turn this converted stem cell back into a blood cell than, say, a brain cell. A different technique for creating stem cells, which controversially involves destroying embryos, doesn’t share this problem.

A natural experiment in Nicaragua clearly shows that language has the ability to affect the way we think. In the 1970s, deaf Nicaraguan schoolchildren spontaneously invented a new sign language with its own grammar and vocabulary. The language has become more complex over time, and so have the thoughts of its users. Jennie Pyers found that people who learned NSL before it developed specific gestures for left and right perform more poorly on a spatial awareness test than children who grew up knowing how to sign those terms.

Imagine taking a course of antibiotics and suddenly finding that your sexual desires have changed. That’s exactly what happened to some Israeli fruit flies, which were raised on diets of either maltose or starch. They developed gut bacteria that specialised on these foods and as a result, they preferred to mate with other flies with similar food biases. A dose of antibiotics, and their mating preferences vanished. The study suggests that you can only understand an animal’s evolution by considering the genes of the bacteria and other passengers that live inside it. This is the hologenome – the combined genes of a host and all the microbes it contains.

Two studies this year showed that humans often have nothing to do with the diseases that plague us; we’re just collateral damage in an invisible war. When faced with predatory amoebas, E.coli (a mostly harmless gut bacteria) turns nasty, evolving into a resistant strain that kills the predators from within. And by coincidence, these same strains are also better at infecting human cells. Meanwhile, the normally harmless nose bacteria Streptococcus pneumonia becomes infectious when it battles against another species called Haemophilius influenzae. This competitor summons white blood cells to do away with Streptococcus, which can defend itself by producing a thicker coat. But this armour also allows Streptococcus to evade our own immune system, resulting in pneumonia, meningitis and other diseases.

This crossfire effect explains why some bacteria cause potentially fatal diseases, even though they’re not contagious. They go down with their host, because they are not really targeting their host at all. Many human diseases really have nothing to do with us at all. However, some scientists are turning these bacterial wars to our advantage, harnessing the weapons that they use against one another.